The vast ice sheet covering Greenland is thinning by up to a metre a year, according to a paper in this week's edition of the journal Science.

The discovery was made by aerial surveyors from NASA, who say the thinning poses a potential threat to coastal communities around the world.

The Greenland ice sheet is the world's second largest after Antarctica. It is more than three kilometres thick and covers an area of 2 million square kilometres.

Nasa used aircraft equipped with laser altimeters to measure the profile of the ice. Their first survey was conducted five years ago. Last year they went back to monitor any changes.

Accurate comparison

The global positioning system of satellites and other navigational equipment helped them to pilot their planes along the same flight path 400 meters above the icy surface. This ensured the 1993 and 1998 data sets were comparable.

They found the sheet had lost up to five metres in thickness. On the west side of the sheet, some areas grew while others shrank - essentially balancing out.

But there was substantial thinning almost everywhere on the east and south sides of the ice sheet, leading to an overall loss of Greenland ice.

Bill Krabill, from Nasa's Goddard Space Flight Centre study, who co-wrote the report, says the thinning was far worse than expected, although the reasons for it are not clear.

Water slippage

Global warming could be partly responsible, with fewer heavy snowfalls and more melting in summer.

But the researchers think there is a more likely explanation: they think water is filtering down to the bedrock, creating a smooth layer underneath the ice making it more likely to slip off into the ocean.

"It's probably slipping faster into the ocean than it has in the past," says Bill Krabill. "This isn't something we will have to worry about in our lifetime, but it will be a question for our children if this phenomenon is accelerating,"

The thinning has serious implications for the rest of the world.

Global implications

Melting ice on the east coast of Greenland helps drive the Earth's ocean currents, which keep Europe warm in winter and affect rainfall and climate worldwide.

Upsetting the melting rate could slow or even stop those currents altogether. The extra melted ice could also raise sea levels. However, all these processes are extremely complex.

Some warming in the world's climate could actually increase precipitation in the Polar regions, reducing the likelihood of a large sea rise.

Even given this latest research, it is currently still not clear whether the overall mass of the world's polar ice sheets is growing, shrinking, or holding firm.